1. Speciation
Proposing Mechanisms for Species Formation and Identifying Ways to Maintain Reproductive Isolation

2. Two Types of Evolution Microevolution Macroevolution
Change within a population or species
Change to a new species
Macroevolution

3. Species Formation
Species = Group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups
Speciation depends on
isolation (lack of gene flow)
genetic divergence

4. Mechanisms for Speciation
Allopatric Speciation
Occurs as a result of geographical isolation
Most common mechanism
Sympatric Speciation
Occurs in the same location
Can be due to Ecological isolation
Can be due to Polyploidy
Occurs for plants that have a sudden change in numbers of chromosome sets

5. Allopatric Speciation
Single species (white mice); homogeneous habitat
(b)
Geographical barrier (impassable river); isolated populations
(a) A single species (white mice) occupies a relatively homogeneous habitat.
(b) An impassable geographical barrier (here, a river changing course) splits the habitat into two parts, separating the species into two isolated populations.
(c) Genetic drift or different environmental pressures cause the two populations to diverge genetically (tan vs. white mice).
(d) The barrier is removed (the river changes course again), and the members of the two populations can share the same habitat. If the genetic differences between the two populations have become large enough that interbreeding cannot occur (that is, they are reproductively isolated from one another), then the two populations constitute separate species (brown vs. white mice).
(c)
(d)
Genetic drift; genetic divergence; tan vs. white mice
Barrier removed (river dries up); Mice mix but don’t interbreed.

6. Summary of Allopatric Speciation
One group separates from the population.
Separate evolutionary pressures cause different genetic changes in both groups.
(Is this (1) microevolution or (2) macroevolution?)
Sufficient genetic changes accumulate so that interbreeding cannot occur if groups are rejoined.

7. Genetic Divergence After Geographical Isolation
Four races of mole rats show different numbers of chromosomes
Races live in areas with differing moisture content and soil type
Hybrids rarely form
The Israeli mole rat is a fossorial (hole-dwelling) rodent with habits similar to a North American gopher. It occupies an extensive system of underground burrows, and travels above the surface only rarely. Four chromosomal races live in different environments and  inhabit different soil types. The ancestral 2n=52 type inhabits the cold, arid Golan Heights of the Trans-Jordan region in the northeast. The Jordan River serves as an effective barrier to gene flow with the 2n=54 type in the moister coastal areas of northern Israel and southern Lebanon. The 2n=58 type inhabits the temperate areas of Samaria. The 2n=60 type inhabits the hot, dry Negev Desert of southern Israel. Hybridization between races is rare an confined to very narrow hybrid zones between races (dark squares).

8. Adaptive Radiation
One species gives rise to many new species in a short period of time
populations invade a variety of habitats and evolve in response to differing environmental pressures in those habitats
results from
availability of unoccupied habitats
lack of competition

9. Sympatric Speciation (a)
Single species (white mice); homogeneous habitat
(b)
Climate change; two habitats; isolated because don’t mix
(a) A single species (white mice) occupies a homogeneous habitat.
(b) Climate change or other factors form two distinctly different habitats that are still physically part of the same general region; that is, there are no barriers to movement between habitats.
(c) Different environmental pressures in the two habitats lead to genetic divergence of organisms living in each (tan vs. white mice).
(d) Sufficient genetic divergence causes reproductive isolation; former occupants of the two different habitats are now separate species (brown vs. white mice).
(c)
(d)
Environmental pressure to adapt; genetic divergence; tan vs. white mice
Sufficient divergence; now different species

10. Speciation by Polyploidy
Diploid with chromosome set A and chromosome set B.
Chromosomes duplicate but do not separate Tetraploid with two sets of A and B.
Cross between diploid and tetraploid species  Triploid with one each of
chromosome sets A, B and D.
Modern Wheat
Chromosomes duplicate but do not separate 
Hexaploid with three sets of A, B and D.

11. Fertility of Polyploids
Tetraploid Plant
Tetraploid Plant
Diploid Plant
Meiosis
Meiosis
Diploid Gamete
Haploid Gamete
Chromosomal mutations, especially in plants, can result in polyploid individuals with extra copies of each chromosome. This example shows that a tetraploid mutant can successfully self-fertilize (or can interbreed with other tetraploid individuals) to yield a new generation of tetraploids but that matings between tetraploids and normal diploid individuals will yield only sterile offspring. Tetraploid mutants are thus reproductively isolated from their diploid ancestors and may constitute a new species.
Diploid Gametes
Fertilization
Fertilization
Viable Tetraploid Zygote
Viable Triploid Zygote
Meiosis
Meiosis (faulty)
Meiosis yields unbalanced products no viable zygotes.
Diploid Gametes

12. Applying Your Knowledge
Sympatric Speciation
Speciation by Polyploidy
Allopatric Speciation
Which process involves a sudden, large change in chromosome number?
Which process requires geographical separation?
Which process can occur as a result of small differences within the same local environment?

13. Maintaining Reproductive Isolation
Premating Isolating Mechanisms
Prevent mating between individuals of different species
Postmating Isolating Mechanisms
Interfere with development, viability or fertility of offspring from matings between species

14. Premating Isolating Mechanism: Geographical Isolation
The Grand Canyon is a geographical barrier that can promote speciation.
The Kaibab squirrel (left) lives on the North Rim while the Avert squirrel (right) lives on the South Rim.

15. Premating Isolating Mechanism: Ecological Isolation
White-crowned Sparrow inhabits fields and meadows
White-throated Sparrow feeds in dense thickets

16. Premating Isolating Mechanism: Ecological Isolation
Each of the 750 species of fig wasps pollinates a different species of fig plant.

17. Premating Isolating Mechanism: Temporal Isolation

18. Premating Isolating Mechanism: Temporal Isolation
Monterey Pine releases pollen in early spring
Bishop’s Pine releases pollen in summer

19. Premating Isolating Mechanism: Temporal Isolation
Periodic Cicada: Males emerge on a 13 or 17-year cycle.

20. Premating Isolating Mechanism: Behavioral Isolation
Fruit fly species have different wing beat frequencies for the mating ritual.
Firefly signaling patterns differ between species

21. Premating Isolating Mechanism: Behavioral Isolation
White-crowned sparrow (Zonotrichia leucophrys) songs are used in male territorial displays, and vary across short distances. The sonograms show frequency against time for three individuals from each of two populations in Northern California. The Berkeley dialect is a rising two-note call followed by a trailing trill, the Marin dialect is a single note followed by a steady trill. Song dialects appeared to be learned. (after Jonsson 1992; from Futuyma 1998).
White-crowned sparrows from different areas
have distinct song patterns for territorial displays.

22. Premating Isolating Mechanism
Examples
Mechanical Isolation:
differences in structure of sexual organs
-Male sex organs of separate insect species vary greatly -Pollen from one plant species does not germinate on female part of another species
female
clasper

23. Postmating Isolating Mechanism
Example
Gametic Incompatibility
-Sperm may not have the proper enzymes to remove the egg’s protective coat
- Pollen may not be able to grow toward the egg in plants of different species

24. Postmating Isolating Mechanism: Hybrid Inviability
Hybrids between Sheep and Goats do not survive

25. Postmating Isolating Mechanism: Hybrid Infertility
horse +
Mule
Cannot Reproduce
donkey

26. Postmating Isolating Mechanism: Hybrid Breakdown
Liger
The liger hybrid is more common than the tigon because the mating process is easier. The liger has both stripes and spots. The stripes are inherited from its tiger parent and the spots from the lion parent. Ligers are usually orangish/golden in color. However, there have been white tigers bred with lions to produce a very light golden coat on the offspring. If the hybrid offspring is a male, it will have a leonine mane, but it will not be as large and defined as a normal lion's mane. The liger gets most of its strength and size from both of its parents; this makes the liger possibly the largest cat in the world! On their hind legs, ligers stand approximately 12 feet tall. At most, ligers may weigh up to 1,000 pounds. Ligers make the sound of both a lion and a tiger, although their roar tends to sound more like a lion's roar.
Tiglion
Hybrids of Lions and Tigers have been obtained through zoo breeding programs. While the first generation may be healthy, hybrids become weak or sterile in future generations.

27. Applying Your Knowledge
Premating Isolating Mechanism
Postmating Isolating Mechanism
Hybrids of dogs and wolves can occur. Some have more dog traits and others favor wolves. After two or more generations, the hybrids are feeble and cannot reproduce.
The male fruit fly flaps his wing to attract a nearby female. Wing beat frequencies differ between species so a female will only accept the male that has the correct wing beat.

28. Applying Your Knowledge
Temporal Isolation
Ecological Isolation
Behavioral Isolation
Geographical Isolation
Mechanical Isolation
English Oak is found in Europe and Scrub Oak is found in California.
Lions live in groups called prides on open grassland. Tigers are solitary and live in forests. These species do not form hybrids even though their ranges overlap.
The male Reggiana bird of paradise displays his feathers to a potential mate. The female trumpet manucode, a related species, is not attracted by this display.

29. Extinction Death of all members of a species
Factors that contribute to extinction
localized distribution of a species
overspecialization
competition
habitat change and destruction

30. Localized Species Distribution
The Devil’s Hole Pupfish lives in a single pool at the mouth of a water-filled
cave system in Nevada.

31. Overspecialization
The Everglades Kite depends on a single species of snail as a food source.